GCSE Physics

Electric Circuits

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Units

• Use the following units: ampere (A), coulomb (C), ohm (Ω), volt (V), watt (W), kilowatt-hour (kWh)

Charge and Current

• Recall that current is rate of flow of charge
• State and be able to use the equation charge = current x time (Q = I × t)
• Recall that electric current in molten or dissolved electrolytes is a flow of both (a) negatively charged ions to the positive terminal and (b) positively charged ions to the negative terminal. (An ion is an atom or a group of atoms that has extra electrons or has had some electrons removed.)
• Electrons do not travel through solutions.
• Recall that electric current in solid metallic conductors is a flow of negatively charged electrons from negative to positive.

Electric Circuits

• Know that cells, batteries and generators are parts of a circuit where electrical energy is produced, i.e. they are electrical sources. (In cells and batteries, chemical energy is transformed to electrical energy).
• Know that other components - namely bulbs, resistors, bells, motors, LEDs, LDRs, thermistors and buzzers - are parts of an electrical circuit where electrical energy is transformed and to dissipated as some other kind of energy such as heat, sound, light etc.
• Understand that the current in a series circuit depends on (a) the applied voltage and (b) the number and type of the components in the circuit.
• A current will flow through an electrical component only if there is a voltage or potential difference (p.d.) across its ends. The bigger the potential difference across the component, the bigger the current that flows through it.
• Components resist a current flowing through them. The bigger their resistance, the smaller the current produced by a particular voltage.
• The p.d. across a component in a circuit is measured in volts (V) using a voltmeter connected across (in parallel with) the component.
• The current flowing through a component in a circuit is measured in amperes (A) using an ammeter connected in series with the component.
• Resistance is measured in ohms (Ω).
• Measure resistance by correctly placing a voltmeter and ammeter in a circuit.
• Know the electrical symbols for a cell, battery, power supply, filament bulb, switch, LDR, fixed resistor, variable resistor, fuse, diode, LED, motor, heater, thermistor, ammeter and voltmeter.
• Recall that components with resistance are heated when electricity passes through them.
• Describe and explain the effect of a variable resistor in controlling the brightness of a lamp and the speed of a motor.
• State and be able to use the equation V = I R. The current through a resistor (at constant temperature) is directly proportional to the voltage across the resistor.
• The current through a diode flows in one direction only. The diode has a very high resistance in the reverse direction.
• Sketch graphs of how current varies with voltage in (a) a metal wire or an ordinary resistor at constant temperature, (b) a filament bulb and (c) a silicon diode.

• The resistance of a filament lamp increases as the temperature of the filament increases.
• The resistance of a light dependent resistor (LDR) decreases as the light intensity increases.
• A thermistor (short for thermal resistor) is a special resistor that is particularly sensitive to temperature. It exhibits a large change in resistance with a small change in its temperature.
• The resistance of a thermistor decreases as the temperature increases [for the type covered by the syllabus. There are some thermistors where the reverse is true]

• When components are connected in series:

  • their total resistance is the sum of their separate resistances;
  • the same current flows through each component;
  • the total potential difference of the supply is shared between them.

• When components are connected in parallel:

  • there is the same potential difference across each component;
  • the total current through the whole circuit is the sum of the currents through the separate components.

• Explain whether a series or parallel circuit is more appropriate for a range of applications, including domestic lighting.

• The potential difference provided by cells connected in series is the sum of the potential difference of each cell separately (bearing in mind the direction in which they are connected).

Energy in Circuits

• As an electric current flows through a circuit, energy is transferred from the battery or power supply to the components in the electrical circuit.
• The higher the voltage of a supply, the greater the amount of energy transferred for a given amount of charge which flows.
• When electrical charge flows through a resistor, electrical energy is transferred as heat.
• Recall that the voltage between two points is the number of joules of energy transferred for each coulomb of charge that passes between the points. Thus
  1 volt = 1 joule per coulomb and energy transferred = potential difference × charge
• The rate of energy transfer (power) is given by: power = potential difference × current.
  Thus 1 watt is the transfer of 1 joule of energy in 1 second.